1. Field of the Invention
This invention relates to a solar cell of amorphous silicon capable of converting solar radiation into electrical energy.
2. Description of Prior Art
Of late, solar cells of amorphous silicon have been watched as a low cost solar cell capable of utilizing clean and inexhaustible energy. Solar cells of amorphous silicon have a feature that the cost of production thereof is low because of being made up of a low-priced substrate and a thin film of less than 1 .mu.m in thickness, formed by a low temperature process. In order to make the best use of this feature of low cost, it is necessary to raise the photovoltaic conversion efficiency of an amorphous silicon solar cell so that it can be put to practical use.
Various proposals have hitherto been made to improve the performances of a solar cell using amorphous silicon. Many of solar cells of PIN structure are so constructed that a P type layer is provided at the illumination side so as to collect readily holes with a shorter diffusion length of pairs of electron-hole formed in high density by solar radiation and to raise the comprehensive collection efficiency of solar radiation.
However, a P type layer containing a Group IIIb element of Periodic Table such as boron (B) exhibiting P type has generally such a large light absorption coefficient that a lot of photons cannot be fed to an I layer where carriers are formed. Therefore, in a PIN structure in which a P type layer is arranged at the illumination side, amorphous silicon to which carbon or nitrogen with a larger forbidden band gap is added (a-Si:C:H or a-Si:N:H) is used for the P type layer to utilize effectively lights of short wavelengths. Furthermore, if the fill factor is improved, the photovoltaic conversion efficiency can further be increased. In this case, it can be expected to improve the series resistance of a solar cell, to improve the fill factor and to raise the photovoltaic conversion efficiency by decreasing the electric resistance of an interface between a P type layer and electrode layer.
U.S. Pat. No. 4,064,521 describes one example of a PIN solar cell including a body of amorphous silicon comprising a P type first layer doped with boron, an intrinsic layer and N type second layer doped with phosphorus, but in this case, the P type layer in contact with a transmissive electrode is uniformly doped. Thus, this solar cell has an insufficient photovoltaic efficiency.